5,000 users in a niche community would need hundreds of millions in the wider network.

This is how bulletin boards used to work. The most successful were focused on a niche and one with 5,000 users would be big enough to be of use to people interested in that niche. But when your niche is part of a much larger community covering all niches, that community needs to be vast to get 5,000 subscribing to any given niche.